Light emitting devices including Light Emitting Diodes (LEDs) are widely applied to display devices indicator lamps, various switches, signaling devices, and optical devices like general lighting instrument for indoor, outdoor, stationary and moving locations, etc. In such light emitting devices that include LEDs, a light transmissive light emitting device which has a plurality of LEDs laid out between two light transmissive substrates has been known as a device suitable for display devices and indication lamps, etc., that display various character strings, geometric figures and patterns.
By applying a flexible substrate, etc., formed of a light transmissive resin as the light transmissive substrate, a constraint to an attachment plane for the light emitting device as the display device, the indication lamp, etc., is eased. This improves the convenience and applicability of the light transmissive light emitting device.
The light transmissive light emitting device includes, for example, a first light transmissive insulation substrate that has a first conductive circuitry layer, a second light transmissive insulation substrate that has a second conductive circuitry layer, and, a plurality of LED chips laid out therebetween. The plurality of LED chips each includes an electrode pair, and such electrode pair is electrically connected to the respective first and second conductive circuitry layers. A space between the first light transmissive insulation substrate and the second light transmissive insulation substrate produced by the plurality of LED chips laid out with a certain clearance is filled with a light transmissive insulator formed of a light transmissive resin, etc., which has the electrical insulation property and the flexibility. In other words, the LED chips are laid out in a through-hole formed in the light transmissive insulator.
As for the electrical connection between the electrode of the LED chip and the conductive circuitry layer in the above-explained light transmissive light emitting device, for example, thermal compression bonding is typically applied to a laminated body that includes the first light transmissive insulation substrate, the light transmissive insulation resin sheet which has the through-hole in which the LED chips are laid out, and the second light transmissive insulation substrate. In this case, by designing the thickness of the light transmissive insulation resin sheet (thickness of light transmissive insulator) having undergone the thermal compression bonding to be thinner than that of the LED chip, the conductive circuitry layer is depressed against the electrode of the LED chip so as to be in contact therewith. The electrode of the LED chip and the conductive circuitry layer may be bonded by a conductive adhesive. In addition, a technology of providing a hot-melt adhesive sheet that fastens the LED chips between the upper and lower insulation substrates each including the conductive circuitry layer, performing thermal compression bonding thereto to embed the LED chips in the adhesive sheet, thereby simultaneously accomplishing the bonding between the upper and lower insulation substrates and the electrical connection between the electrode of the LED chip and the conductive circuitry layer has been proposed.
In any cases, however, the electrical connection between the conductive circuitry layer and the electrode, and the reliability thereof are not sufficiently ensured. Therefore, there is a need to improve those properties.